Planar lightwave circuit package

ABSTRACT

An optical packaging arrangement combines a planar lightwave circuit (PLC) having an array of waveguides thereon, an array of photodetectors on a substrate to receive light beams coupled out of the PLC by the output ports, and a collimating faceplate, having a plurality of glass cores, extending between the PLC and the photodetector array for coupling the output light beams to respective photodetectors. The faceplate forms a cover for a hermetic cavity encompassing the photodetectors. The PLC is disposed either co-planarly with the faceplate or transversely to it. Light from the PLC is tapped via a plurality of taps formed on the PLC for coupling to the photodetectors.

RELATED APPLICATION

none

FIELD OF THE INVENTION

This invention relates generally to optical data communication devicesand more specifically, to a packaging arrangement for opticaltelecommunications, the arrangement including an array of optical inputports, e.g. a planar lightwave circuit, and a plurality ofphotodetectors, the optical input array being optically coupled with thephotodectors.

BACKGROUND OF THE INVENTION

Planar lightwave circuits (PLC) are well known in opticalcommunications. They are formed on various substrates and include anetwork of waveguides, for example channel waveguides. An example of aPLC is shown in U.S. Pat. No. 6,507,680 issued Jan. 14, 2003 toNishimura et al.

In the design of PLC architectures, it is important to properlyintegrate the taps and photodetectors with the optical transmissionchannels e.g. channel waveguides. In Nishimura, the photodetectors areintegrated with the PLC and arranged for evanescent coupling.

It is also known to mount a photodetector in the path of a light beamfor direct detection of a (tapped) light beam incident on thephotodetector.

It is important to keep the photodetector(s) in a hermetically sealedhousing as contaminants such as dust particles, water vapor orcondensate, dust, fumes, smoke and other pollutants can adversely affectthe photodetector's performance.

It is also desirable, when designing a planar lightwave circuitarrangement including a separate (i.e. not integral) photodetector (PD)or a PD array, to provide a spacing between the PLC and the PD array.The spacing should be sufficient to prevent a direct contact between thePLC and the photodetector array, but not excessive to avoid anundesirable divergence of a light beam tapped out of the PLC towards thePD array. The current embodiment utilizes a 200 μm spacing between thecollimating plate and the photodetector array. This allows light to fillmost of the photodetector element which is typically 80 μm in diameter.The light emerges from the waveguide at approximately 8 μm in diameterwith a divergence angle of 12 degrees. The spacing between thecollimating plate and photodetector allows the light to diverge to a 60μm spot, thus filling most of the photodetector element. Thesedimensions can change depending on the photodetector element diameterselected. It is always desirable to fill at least a major part of thephotodetector element, irrespective of the selected element nominaldiameter. The secondary advantage of spacing the collimating plate fromthe photodetector array is that it prevents mechanical stresses from theoutside surface of the collimating plate from being impeded into thephotodetector array.

In an arrangement where the PD array is not an integral part of the PLC,the hermeticity requirement can be met by designing casings encompassingthe entire PLC arrangement, i.e. the PLC with taps and the photodetectorarray. This however is a relatively costly solution. It is desirable toreduce the cost of a hermetic arrangement of the above-discussed typewithout sacrificing the hermeticity of the package and the quality ofoptical coupling between the PLC (specifically, the optical taps) andthe respective photodetectors.

The prior art includes various examples of coupling between opticalwaveguides and photodetectors. U.S. Pat. No. 5,586,207 issued Dec. 17,1996 to Northern Telecom describes methods and assemblies for packagingoptoelectronic devices including a method of coupling an optical fiberto a packaged device using a collimating faceplate composed of parallelsections of optical fibers.

Collimating faceplates are also used in other arrangements, e.g.described in U.S. Pat. No. 6,160,606 to Sprague; U.S. Pat. No. 6,137,929to Rosenberg et al; 6,318,909 to Giboney et al.; U.S. Pat. No. 5,170,455to Giboney et al; U.S. Pat. No. 5,170,455 to Goossen et al; and WO02/39155 published May 16, 2002.

SUMMARY OF THE INVENTION

In accordance with the invention, there is provided a packagingarrangement including a fixed array of optical input ports, for examplea planar lightwave circuit (PLC), a liquid crystal display or a diodelaser array. In the embodiment where the array is a PLC, the PLC definesat least one major surface having an array of waveguides thereon. Thearray of waveguides also includes one or more output ports e.g. opticaltaps or equivalent light diverting means for coupling light beamspropagating in the PLC, or their portions, out of the PLC.

The package of the invention further comprises an array ofphotodetectors disposed on a photodetector substrate to receive lightbeams coupled out of the array, e.g. a PLC, by the output ports, and acollimating faceplate extending between the PLC and the photodetectorarray for coupling the output light beams to respective photodetectors.The faceplate is dimensioned and configured to form, along with thephotodetector substrate, a hermetic cavity encompassing thephotodetectors.

The definition “fixed array” denotes a permanent array with apredetermined spatial relationship between the components of the array,e.g. laser diodes, or waveguide outputs of a PLC, as opposed to a loosebundle of optical fibers.

In one embodiment of the invention wherein the fixed array of inputports is a PLC, the faceplate is planar and the PLC is disposedco-planarly therewith, i.e. with its major surface contiguous with onesurface of the faceplate while the opposite surface of the faceplate isdisposed to face the photodetectors for the coupling of light beams, ortheir portions, propagating in the PLC, to the photodetectors.

In another embodiment of the invention, the PLC is disposed transverselyto the faceplate, i.e. with a side wall of the PLC facing the faceplate.

The PLC may be spaced from the faceplate, preferably by a distance notimpairing the coupling of light signals from the PLC to thephotodetectors. Alternatively, the PLC may be in contact with thefaceplate, directly or with an adhesive joint using a light-transmissiveadhesive such as an index-matched epoxy adhesive.

The PLC may be embodied by a known arrayed waveguide grating (AWG)module.

It is a feature of the invention that the faceplate and a photodetectorcarrier can form a hermetic cavity while allowing optical energy toreach the photodetector array through the transparent cores of thefaceplate. The faceplate thus offers a sealing function in an economicalmanner (without necessitating a hermetic enclosure over the entire PLCcircuit) while also providing transparency for collimated lighttransmission from the PLC, either through free-space or direct coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in more detail by way of thefollowing description in conjunction with the drawings in which likereference numerals represent like elements and in which

FIG. 1 is a schematic plan view of a conventional planar lightwavecircuit (PLC),

FIG. 2 is a schematic cross-sectional view of a conventional collimatingfaceplate,

FIG. 2A is a plan view of the collimating faceplate of FIG. 2,

FIG. 3 is an exploded cross-sectional view of an embodiment of the PLCpackaging assembly of the invention,

FIG. 4 is a perspectve view of the embodiment of FIG. 3,

FIG. 5 illustrates an alternative arrangement of the PLC assembly of theinvention, and

FIG. 6 illustrates an embodiment in which the PLC is an arrayedwaveguide grating (AWG).

DETAILED DESCRIPTION OF THE INVENTION

Turning first to FIG. 1 (prior art), a typical planar lightwave circuit(PLC) 10 is shown schematically. The PLC has a planar substrate 12, aplurality of input channel waveguides 14 a-14 n and a plurality ofoutput channel waveguides 16 a-16 n. Each input waveguide is providedwith a 5%-95% coupler 18 thus defining 95% arms extending into outputwaveguides 16 a-16 n, and a plurality of 5% arms 20. The 5% arm 20 ofevery coupler ends with a tap 22. The taps are formed by metallizedmirror surfaces disposed at an angle, approximately 45°, to thepropagation axis of a light beam propagating in the 5% arm 20.

The taps 22 can be made for example by etching the angled reflectivesurfaces into the waveguide followed by depositing gold onto thereflective surface using a sputtering process.

In an embodiment of the invention, the taps are created by RIE (reactiveion etching) of a 45-degree notch through the cladding and core,followed by deposition of a reflective metal coating on the oppositeside of the notch, thus producing a reflective surface for directing thelight into the collimating faceplate.

Alternatively, the taps can be realized by partly transparent, partlyreflective mirror surfaces provided at an angle (e.g. 45°) in the pathof optical beams propagating in the channel waveguides to couple apredetermined portion of light propagating in the respective waveguidesout of the PLC to be detected as described below.

The mirror surfaces may be mounted in grooves, e.g. V-grooves providedacross the path of the respective waveguides in the planar substrate.

Turning now to FIG. 2, a conventional collimating faceplate is shown.Functionally, as well known in the art, the faceplate is a plurality ofglass (or another transparent material) fiber cores 30 of a diameterfrom approx. 3 to 10 μm, the cores having a relatively high numericalaperture and being embedded in a transparent, partly transparent oropaque material 32 that serves as a cladding to the core. The refractiveindex of the cores 30 should be higher than the refractive index of thecladding material to enable the total internal reflection of any lightentering the core 30.

FIG. 2A illustrates the faceplate of FIG. 2 in a plan view showing amajor surface 40 of the faceplate with the shaded areas denoting thecores 30 and cladding areas 32 between the cores 30.

An embodiment of the packaging arrangement of the present invention isshown in FIG. 3. A ceramic photodetector carrier 50, shown without itsfront wall, houses a commercially available InGaAs photodetector array52. The array has a plurality of separate photodiodes, at leastcorresponding in number to the number of taps (not shown in FIG. 3) onthe associated planar lightwave circuit 54. The upper major surface ofthe circuit 54 extends along the, cavity defined by the walls of thecarrier 50 and is separated from the cavity by a collimating faceplate56. The faceplate 56, approximately 1.2 mm in thickness, is at leastco-extensive with the bottom walls 58 of the carrier 50 and is shaped tomatch the walls 58 to provide a sealed cover for the cavity 60 of thecarrier 50. In a simplest embodiment, the faceplate has a planar surfacefacing the cavity and the bottom walls 58 of the carrier 50 define aplane, but other surface configurations are also feasible provided thatthe faceplate 56 matches the bottom walls to provided a sealing coverfor the cavity 60.

The faceplate may be manufactured according to any of known techniques,for example as per the U.S. Pat. No. 6,160,606 (Sprague).

The cavity 60 housing the photodetector array 52 may be filled with airor another gas, e.g. an inert gas, and should be free from deleteriouscontaminants jeopardizing the functioning of the photodetector array asexplained above.

The planar lightwave circuit 54 includes a number of channel waveguidesas illustrated in FIG. 1. Only one waveguide section (arm) 20 is shownschematically in FIG. 3. A V-groove 70 is formed by etching in the PLC54 in the path of the arm 20. The left-side wall of the V-groove istransmissive, while the right-side wall, disposed at 45° to thepropagation axis of the arm 20 has a reflective surface formed bysputtering of a metal layer 73. Thus, light propagating from the left inthe waveguide arm 20 passes through the left wall of the V-groove andreflects from the right wall of the V-groove upwards i.e. towards thephotodetector array via a core area 72 (indicated in phantom lines) ofthe faceplate 54.

Alternatively, it is possible to apply a reflective coating onto theoutput facet (edge) of the waveguide where light would normally emergeout of the waveguide. This would result in the light returning backthrough the original optical path formed by the waveguides. A 50%splitter could be applied near the input of the waveguide at which pointthe collimating photodetector package could be placed.

An exemplary finished PLC package, or assembly, of the invention isrepresented schematically in FIG. 4. For more effective sealing of thefaceplate to the photodetector carrier, a conventional sealing compoundor alloy, e.g. a known Au/Sn alloy, may be applied at the matchingsurfaces of the faceplate 56 and the carrier 50. The photodetector array52 is thus sealed in the cavity 60, with its electric leads (notillustrated) arranged in a manner not affecting the hermetic seal.

The components of the assembly illustrated in FIG. 4 are dimensionedsuch that the spacing between the faceplate 56 and the photodetector 52is small, e.g. about 0.2 mm, to limit the effect of the inevitabledivergence of light emerging from the faceplate towards thephotodetector, as represented by the dashed lines in FIG. 4. On theother hand, the spacing should not be too small to alleviate the risk ofcontact between the faceplate and the photodetector with associatedpossibility of losing the seal between the faceplate and thephotodetector substrate and a damage to the photodetector.

The PLC 54 may be positively attached to the faceplate 56 using anadhesive, e.g. a known UV (epoxy) adhesive, having a refractive indexmatched to the refractive index of the cores of the faceplate. Tominimize light losses in the adhesive, a bond line thickness of about 10μm or less is used.

In an alternative embodiment of the invention represented schematicallyin FIG. 5, the PLC 54 is disposed differently than in FIG. 4. The majorsurface of the PLC 54 with the waveguides 20 is disposed transversely orapproximately orthogonally (in a wide range of angles) relative to themajor surface 80 of the faceplate 56, so that the PLC faces thefaceplate with its side rather than the major surface. This arrangementallows for one or more of the output ends 82 of the waveguides 20, theends defining output ports of the respective waveguides, to be alignedwith respective core areas of the faceplate 56 and thus with respectivephotodetectors, without using taps. In the embodiment illustrated inFIG. 5, the waveguides 20 terminate with output ends (serving as inputports within the meaning of the present invention) such that the entiresignal propagating in one of the waveguides is coupled out of the PLCthrough the output end (port) 82. Alternatively, the waveguides may havecouplers as illustrated in FIG. 1 except that all the coupler arms mayterminate at the same edge of the PLC. Such an arrangement isillustrated in FIG. 6 where the PLC 54 of FIG. 5 is represented by anarrayed waveguide grating (AWG) 90 arranged with its major surface 92perpendicularly to the faceplate 56. The photodetector array isindicated schematically as 94. The AWG, arranged as a demultiplexer, hasan input waveguide 96, a slab waveguide 97, an array waveguide portion98, a slab waveguide 99 and a plurality of output optical waveguides100. The output optical waveguides, or at least some of them, havedirectional couplers 102 serving as taps, wherein a tapped portion oflight propagating in the output waveguide is directed to a respectivephotodetector in the array 94, while the rest of the light is coupled toa destination, not shown. The destination may be the edge of the PLC,where the light will be coupled into a fiber or into a free spaceoptical device, not illustrated.

The collimating plate allows a majority of each photodetector element tobe filled with light while not over-filling to the point of losing lightor causing adjacent photodetector elements to pick up light from asingle source.

The spacing between the collimating plate and the photodetectors can beselected to enable the use of photodetector arrays with varying elementdiameters. Closer spacing can be used to create a smaller light spotsize for smaller photodetectors. Larger spacing can be used to filllarger elements in photodetector arrays where higher sensitivity isneeded. Smaller diameter PD elements tend to be less sensitive but offergreater speed and lower electrical noise.

The use of the collimating plate provides a highly reliable hermeticpackage window without affecting the light beams that are directedtowards the photodetector elements. The collimating faceplate preventsexcessive beam divergence or beam angle changes which are key advantagesin the packaging of photodetector arrays designed for the purpose ofmonitoring light emerging from multiple waveguides in a PLC cuircuit.

The collimating faceplate can also enable free-space optical coupling toa photodetector array where direct bonding to a PLC is not an option.This can be done by placing the collimating faceplate very close (e.g.in a range of a few μm to a few hundred μm) but not in contact with thephotodetector array. Up to 200 μm of spacing can then be allowed betweenthe outside surface of the collimating plate (package) and thefree-space light source. This spacing must be adjusted to match thephotodetector element diameter in a way that is similar to the tuningtechniques described hereinabove. As indicated above, the inventionapplicable to various arrays of optical input ports, such as liquidcrystal displays (LCD) or laser diode (LD) arrays. FIG. 5 representsschematically, mutatis mutandis, such other arrangements wherein theelement 54 represents a specific fixed array of optical input ports.

The invention eliminates the cumbersome fiber pigtailing tophotodetectors. By controlling the divergence at the PD array throughcollimation by means of the faceplate, the spacing of the photodetectorsin the PD array can be relatively close to accommodate closely spacedmonitor waveguides on the associated PLC.

Of course, numerous other embodiments may occur to those versed in theart without departing from the scope of the invention as defined by theappended claims.

1. An optical circuit assembly comprising: a fixed array of opticalinput ports, a photodetector substrate defining a cavity, aphotodetector array mounted in said cavity and disposed to receive atleast one optical signal from the fixed array of input ports, and afaceplate comprising a plurality of substantially parallel optical fibercores, the faceplate optically coupled between the fixed array and thephotodetector and arranged such that at least some of said cores arealigned with the input ports and with the photodetector array, whereinthe faceplate is disposed and dimensioned to define a sealing cover ofthe cavity wherein the fixed array is a liquid crystal display.
 2. Anoptical circuit assembly comprising: a fixed array of optical inputports, a photodetector substrate defining a cavity, a photodetectorarray mounted in said cavity and disposed to receive at least oneoptical signal from the fixed array of input ports, and a faceplatecomprising a plurality of substantially parallel optical fiber cores,the faceplate optically coupled between the fixed array and thephotodetector and arranged such that at least some of said cores arealigned with the input ports and with the photodetector array, whereinthe faceplate is disposed and dimensioned to define a sealing cover ofthe cavity wherein the fixed array is a diode laser array.
 3. A planarlightwave circuit assembly comprising: a planar lightwave circuit (PLC)comprising at least one waveguide for propagating a light signaltherethrough and an output port disposed for directing at least a partof the light signal out of the PLC, a photodetector substrate defining acavity, a photodetector array mounted in said cavity and disposed toreceive at least a part of the light signal from the output port, and afaceplate comprising a plurality of substantially parallel optical fibercores, the faceplate optically coupled between the planar lightwavecircuit and the photodetector and arranged such that at least one ofsaid cores is aligned with the output port and with the photodetector,wherein the faceplate is disposed and dimensioned to define a sealingcover of the cavity wherein the PLC is disposed parallel to thefaceplate wherein the planar lightwave circuit defines a major surfaceand comprises at least one optical tap disposed to couple at least partof the light signal propagating in the waveguide transversely to themanor surface.
 4. The assembly of claim 3 wherein the optical tapcomprises a V-groove having at least one reflective surface.
 5. A planarlightwave circuit assembly comprising: a planar lightwave circuit (PLC)comprising at least one waveguide for propagating a light signaltherethrough and an output port disposed for directing at least a partof the light signal out of the PLC, a photodetector substrate defining acavity, a photodetector array mounted in said cavity and disposed toreceive at least a part of the light signal from the output port, and afaceplate comprising a plurality of substantially parallel optical fibercores, the faceplate optically coupled between the planar lightwavecircuit and the photodetector and arranged such that at least one ofsaid cores is aligned with the output port and with the photodetector,wherein the faceplate is disposed and dimensioned to define a sealingcover of the cavity wherein a sealing alloy or sealing compound isdisposed at an interface of the faceplate and said photodetectorsubstrate for sealing the cavity.
 6. A planar lightwave circuit assemblycomprising: a planar lightwave circuit (PLC) comprising at least onewaveguide for propagating a light signal therethrough and an output portdisposed for directing at least a part of the light signal out of thePLC, a photodetector substrate defining a cavity, a photodetector arraymounted in said cavity and disposed to receive at least a part of thelight signal from the output port, and a faceplate comprising aplurality of substantially parallel optical fiber cores, the faceplateoptically coupled between the planar lightwave circuit and thephotodetector and arranged such that at least one of said cores isaligned with the output port and with the photodetector, wherein thefaceplate is disposed and dimensioned to define a sealing cover of thecavity wherein the PLC is spaced from the faceplate a distancepermitting free space optical coupling between the PLC and thephotodetector array.
 7. A planar lightwave circuit assembly comprising:a planar lightwave circuit (PLC) comprising at least one waveguide forpropagating a light signal therethrough and an output port disposed fordirecting at least a part of the light signal out of the PLC, aphotodetector substrate defining a cavity, a photodetector array mountedin said cavity and disposed to receive at least a part of the lightsignal from the output port, and a faceplate comprising a plurality ofsubstantially parallel optical fiber cores, the faceplate opticallycoupled between the planar lightwave circuit and the photodetector andarranged such that at least one of said cores is aligned with the outputport and with the photodetector, wherein the faceplate is disposed anddimensioned to define a sealing cover of the cavity wherein the PLC isin contact with the faceplate, wherein an optically transparent adhesiveis disposed between the PLC and the faceplate.
 8. A planar lightwavecircuit assembly comprising: a planar lightwave circuit (PLC) comprisingat least one waveguide for propagating a light signal therethrough andan output port disposed for directing at least a part of the lightsignal out of the PLC, a photodetector substrate defining a cavity, aphotodetector array mounted in said cavity and disposed to receive atleast a part of the light signal from the output port, and a faceplatecomprising a plurality of substantially parallel optical fiber cores,the faceplate optically coupled between the planar lightwave circuit andthe photodetector and arranged such that at least one of said cores isaligned with the output port and with the photodetector, wherein thefaceplate is disposed and dimensioned to define a sealing cover of thecavity, wherein the planar lightwave circuit is an arrayed waveguidegrating (AWG).
 9. A planar lightwave circuit assembly comprising: aplanar lightwave circuit (PLC) comprising at least one waveguide forpropagating a light signal therethrough and an output port disposed fordirecting at least a part of the light signal out of the PLC, aphotodetector substrate defining a cavity, a photodetector array mountedin said cavity and disposed to receive at least a part of the lightsignal from the output port, and a faceplate comprising a plurality ofsubstantially parallel optical fiber cores, the faceplate opticallycoupled between the planar lightwave circuit and the photodetector andarranged such that at least one of said cores is aligned with the outputport and with the photodetector, wherein the faceplate is disposed anddimensioned to define a sealing cover of the cavity, wherein the planarlightwave circuit defines a major surface disposed transversely to thefaceplate, and the output port is disposed at an edge of the majorsurface, wherein the planar lightwave circuit is an arrayed waveguidegrating (AWG).